The present invention is directed to a method for diminishing the trench width in a photoresist structure to below the resolution limit, as well as, to a photoresist system suitable for this purpose.
In the photolithic production of resist structures, the photolithographic properties of the resist material, the exposure wavelength .lambda. employed, and the numerical aperture (NA) of the imaging optics define the resolution limit, i.e., the smallest possible structure (CD) that can be imaged. When both line structures, as well as, trench structures on the mask, can no longer be transferred with sufficient dimensional accuracy into the resist, regardless of their dimension, the resolution limit is surpassed.
As set forth in the following equation, resolution is dependent on a number of physical parameters. ##EQU1## Pursuant to this equation, resolution can essentially only be enhanced by reducing the exposure wavelength or enlarging the numerical aperture. Factor k is process-associated and system-associated, and only exhibits a slight range of variation given systems that are already optimized.
Typical photoresists that ar presently utilized work with an exposure wavelength in the near ultraviolet range (NUV) at 436 or 365 nm. This allows highly resolved structures in the sub-.mu.m range (1 through 0.5 .mu.m) to be achieved. Although there is current development efforts in the area of resists for exposures in deep ultraviolet range (DUV), such a system or product is not yet commercially available.
Currently, only utilizing complicated and expensive methods such as, for example, electron beam or x-ray lithography, can structures in the sub-half .mu.m range (0.5 through 0.2 .mu.m) be produced.
The majority of photoresists used for the sub-.mu.m range function positively. These photoresists are composed of a base polymer and a photo-active component. The photolithographic properties of the resist are characterized by the contrast and by the sensitivity of the material.
Contrast is calculated from the slope of a curve derived from a plot of the chemical erosion of an exposed photoresist layer in the development process versus the logarithm of the corresponding light intensity. A high contrast allows steep edges to be obtained in the photoresist structures at the boundaries between the exposed and unexposed regions of a photoresist layer.
Sensitivity essentially refers to the light sensitivity of a photoresist. It is desirable that sensitivity is enhanced for economic reasons. A higher sensitivity creates a shorter exposure time. Exposure time is the most cost-intensive part of the lithography method.
The photoresist structures are produced on the surfaces of substrates. The surfaces of the substrate can be, for example, semiconductors, ceramic, or metal. The photoresist functions as a mask for the physical or chemical treatment of the substrate surface; the regions that are not covered by the photoresist structure are thereby treated. For example, these regions can have a hole or trench shape.
Due to the miniaturization of microelectronic components, it is necessary to make these resist trenches smaller or narrower. For example, a narrower trench allows the production of a narrower trench cell or a narrower interconnect. In the quest for miniaturization, among the goals is to achieve production-friendly, reproducible, and economical methods, that enhance the resolution limit of the photoresist or to even produce structures that lie beyond the resolution limit of an imaging technique that is used.
Presently, Applicants believe that only two basic methods are currently known for realizing structural dimensions that lie beyond the resolution limit of the lithographic techniques used.
An example of one method is a trench-narrowing technique for an exposure wavelength of 365 nm that is disclosed in a report by Y. Kawamoto et al, "A Half Micron Technology For An Experimental 16 MBIT DRAM Using i-line Stepper", VLSI Symposium, San Diego 1988. The method is based on a three-layer structure wherein the uppermost layer is a top resist. The top resist functions positively. Top resist structures having a width of 0.6 .mu.m are lithographically produced and are enlarged by plasma deposition of silicon dioxide, or the clearance of the resist trenches is reduced. In a plasma etching process that follows, the resist structures are then transferred onto a spin-on glass intermediate layer lying therebelow. Silicon dioxide spacers thereby only remain at the sidewalls of the photoresist structures. The silicon dioxide spacers reduce the width of the photoresist trenches from an original width of 0.6 .mu.m to 0.4 .mu.m. These trenches are transferred into an organic planarization layer lying at the bottom in a further, anisotropic etching process with oxygen plasma.
Utilizing this technique, it is possible, for example, to reduce the memory cell area of an electronic memory module by about 30%, the packing density can thus be correspondingly increased. The method, however, suffers some disadvantages. The disadvantages of the method include the multi-layer structure and the three plasma etching steps required by the method as a result thereof. These steps are complicated to implement and are not particularly production-friendly.
Another method for producing structures below the resolution limit of an optical NUV lithography is described in B. D. Cantos, R. D. Remba, "A Reliable Method For 0.25-Micron Gate MESFET Fabrication Using Optical Photolithography", J. Electrochem. Soc. Acc. Br. Comm., 1311-12 (1988). As set forth therein in a three-layer structure, an image reversal resist is applied as an upper top resist layer and is exposed through a mask. After a tempering step and subsequent flood lighting, resist ridges having a width of 0.25 .mu.m are obtained in the exposed regions by over-development in an aggressive developer. Structures having a width of 0.65 .mu.m are prescribed on the mask. An aluminum layer is then vapor-deposited thereon and "developed" in a lift-off process. By lifting the top resist ridges off, along with an aluminum layer lying thereabove, aluminum ridges having a width of 0.25 .mu.m are obtained in the trenches of the top resist layer. The transfer of this aluminum structure onto an organic planarization layer lying therebelow, together with an insulating intermediate layer, proceeds in an oxygen plasma.
This method also has some disadvantages. Due to the several individual process steps required by the process, the method provides little process tolerance and, thus, is not very production-friendly. Moreover, trench structures only down to a width of 0.25 .mu.m can be obtained through the process.